Engineers and technologist have been seeking for a keyless opening system with a single antenna for years. Many systems have been described in theory, but all of them lack the actual possibility of providing an antenna that overcomes the problem of fragile ferrite magnetic cores.
Keyless entry systems for the automotive industry often work at low frequencies, such as for example, at 20 KHz such as those described in MARQUARDT's patent documents EP-B1-1723615 and WO-A1-2013135381, or at 125 KHz and 134 KHz such as those described in CONTI's patent documents WO-A1-2011120501 or U.S. Pat. No. 9,184,506 B2.
In order to cover a minimum reading distance (capture the wireless signal of the remote control device) from the vehicle, the existing systems usually use short ferrite antennas arranged in the door handles and trunk. These antennas normally use cores made of soft ferrite magnetic material, ZnMn. Since ferrite is a brittle and fragile material, the maximum length of the antennas is limited to a length in which the ferrite can withstand a minimum torque or deformation. This limits the actual length of ferrite cores used to less than 180 mm and typically from 80 to 120 mm. These extremely fragile cores receive a coil which is protected by overmolding or by plastic casings and the antennas made of resulting wire coil are in general embedded in a resin or already overmolded with low pressure or by high pressure polymers.
All these plastic coatings and layers are intended for protecting the fragile ferrite core from external forces, torques, blows and bends.
PREMO's patent application PCT/IB2015/001238 describes flexible magnetic cores and processes for production thereof, based on microwires made from high-permeability soft magnetic alloys and polymer nanoparticles dispersed in a polymer matrix surrounding said microwires.
With continuous ferrite cores, the length of the antennas is limited and the systems of the state of the art describe arrangements with 3 to 5 antennas per vehicle in order to cover a minimum reading distance around the entire vehicle.
Meanwhile, the current antennas used in vehicles generally have a length less than 180 mm, the replacement thereof with a single antenna would require a length between 300 mm and 500 mm in order to generate a magnetic field having sufficient intensity to cover those generated by the current short antennas. However, an antenna so long cannot use a single solid ferrite core because in said case it would break easily with a very small bending force even if it is coated, molded or overmolded by means of a casing or surrounded by a hard plastic casing.
A possibility for solving the mentioned technical problem would be a “keyless” system that uses an elongated, completely flexible, low-frequency LF antenna, such as that provided in PREMO's patent application PCT/IB2015/001238.
This innovation would allow implementing a “KES” system that would provide, with a single antenna, performances equal to or greater than those provided by the systems of the state of the art with 3, 4 or 5 antennas. The innovation leads to a “KES” system having many technical and economical advantages:                The wiring and connectors are reduced by ⅓, ⅕ or ¼, respectively.        OEM assembly time is reduced by ⅓, ⅕ respectively.        The total energy consumption and actual battery losses, a very important parameter particularly in electric vehicles, are proportionally reduced.        A longer antenna requires lower currents to generate equal or more intense magnetic fields, thereby reducing the necessary energy and allowing reducing the cross section of the wire in the antennas' coil.        
A reduction of the electrical output necessary in the vehicle is achieved with the foregoing; MOSFET transistors which allow reducing the number of amplifiers and the characteristics of the power stage by ⅓, ¼ or ⅕ and furthermore allow simplifying the final analog power elements necessary which may be simpler and less expensive due to the fact that a lower current is being used for generating the same magnetic fields than the systems of the art. In general, the Front-End or analog interface of the reader in the electronic control unit (ECU) is simplified both by reduction of the channels that would go from 5, 4 or 3 to 1 and by an important reduction of the power of the remaining channel.
The reliability of a vehicle is proportional to the number of components it incorporates so the mere reduction of the number of antennas and channels in the ECU provides intrinsic reliability increasing the mean time between system failures or MTBF.
Likewise, by using a single antenna the safety elements thereof are simplified.
An elongated inductor comprising multiple ferrite cores has been widely used for AM radio systems. Patent application WO-A2-2009123432 describes a solution consisting of multiple cores of cylindrical rods inside a coil. A more recent application in wireless charging systems was presented by Qualcom in patent application US-A1-2013249303 disclosing a plurality of aligned ferromagnetic elements.
SUMIDA's patent application US-A1-20150295315 describes rigid, solid ferrite cores that are introduced in a coil forming machine with a specific shape for arranging capacitors and waveguides.
Said PREMO's patent application PCT/1132015/001238 describes various materials other than ferrite such as nanocrystalline sheets but they have not been used in practice because said materials have a very significant drawback, the magnetostriction, a property of soft magnetic materials causing great changes in magnetic permeability under pressure or deformation. Therefore, while these sheet materials, although being very expensive, could theoretically be used in long antennas, in practice these antennas do not break, but change their permeability so much that the resonance frequency typical of the tuned tanks that they form in series or capacitors in parallel lack the minimum selectiveness required for a reliable system. On the other hand, the deformation of the sheets is only possible in the axis perpendicular to the wide side, while in the other two orthogonal axes the cores are non-deformable.
PREMO's patent application PCT/IB2015/001238 of PREMO provides an elongated antenna that can be bent in a three-dimensional space both along an X-axis and along an orthogonal Y-axis.
Another solution is described in SUMIDA's patent application US-A1-2015123761 based on a composite core made of a plurality of cylindrical ferrite cores (see FIG. 2) with a spherical concave or convex termination at the head and tail ends thereof, which are coupled to one another, and also discloses cores coupled to another in a book-like configuration (see FIG. 3).
The construction of elongated cores by means of adding longitudinally smaller elements coupled to one another is already disclosed in patent document US-A1-2015123761.
Other documents disclosing composite inductors include U.S. Pat. No. 6,417,665 B1 describing a long magnetometer with a flexible magnetic core, made up of several cores coupled to one another and EP-A2-0848577 describing the construction of a long and flexible magnetic core made of ferrite rods coupled at their ends. Furthermore, coupling at the ends of magnetic cores by means of physical interstices or separations between the spherical or cylindrical contacting surfaces (gap) is a common practice in magnetic rotating machines because they are required for assuring a constant and minimum gap, as well as for free movement. See, for example, the 1974 publication by Bruce De Palma “The generation of a unidirectional Force” (http//depalma.pair.com/GenerationOfUnidirectionalForce.html).
In hard magnetic materials, it is also common practice for moving magnetic parts to use spherical interstices (gaps) in combination with ferrofluids for advanced bearings.
U.S. Pat. No. 7,138,896 describes ferrite cores made of individual elements coupled to one another in a head-tail-head manner by means of a cylindrical gap for EMI (electromagnetic interference) shielding in flat cables operating as an antenna radiating energy in the form of electromagnetic radiation.
The present invention prevents problems in the physical implementation of inductors with a plurality of magnetic cores coupled to one another and affecting their performance when they are applied for constructing an LF antenna for a KES system, as a result of the parasitic vertical and horizontal gaps, in particular:                The discrete cylindrical elements or spherical core elements do not have an adhesive attachment at their contacting articulation ends, and there is no way to assure that the gap, which is demarcated by the distance of air or non-ferromagnetic material between core elements, does not become larger when an elongation occurs in direction X. Therefore, when an elongation occurs in direction X, the distance between the elements increases so the loss of magnetic flux increases, resulting in an increase in magnetic reluctance as a result of a lower permeability, causing a deviation of the resonance frequency and antenna malfunction.        The discrete elements of the core that is cylindrical or having a rectangular cross section can slide with respect to one another, without any retention element between them, thereby causing a horizontal separation generating a misalignment of the magnetic cores coupled to one another increasing the total reluctance in the manner proportional to the number of elements. This horizontal separation or interstice reduces the constant cross section area available intersected by the lines of magnetic field, thereby resulting in a reduction of the effective permeability. On the other hand, the magnetic leakage flux lost in the gap is not redirected by a low reluctance magnetic path, losing the induction capacity in the coil.        
Both effects, i.e., misalignment in the direction of the Y-axis and the enlargement of the gap in the direction of the X-axis of a three-dimensional space determine an inefficient performance of the mentioned composite inductors.
The present invention proposes a solution to the mentioned problems and allows constructing a flexible antenna having a length greater than 300 mm.